Well, let's just say that I'm not as certain as you, but it's not worth arguing over, as what's done is done and there's no way of proving what would have happened.
My father was born and raised near the heart of the most fervent abolitionism and not far from where the foundational meeting of the Republican party was held (and the main topic was advocacy for abolition) and not that far in another direction to a monument to John Brown, so you're not likely to gleen a lot of sympathy from me for the Confederacy. If the Confederacy had its way, my male ancestors on my fathers side would have been slaughtered, imprisoned or cowed and the freed slaves they helped and their black friends hunted down and enslaved. It was Southern slave hunters going into abolitionist towns and catching freed slaves, some of whom were regarded as beloved members of the community, that helped strengthen the abolitionist movement, lead to the Dred Scott Case and eventually help trigger the Civil War. Just one town over from where my father was born, when a slave hunter caught and imprisoned a freed slave, the local townsfolk rose up and liberated the slave. Attempts, both successful and unsuccessful, were also made in other towns and cities to free re-captured slaves, such as Boston, another place where there was an abolition movement, though not as strong as in New York. This is a little-told side of the story of the era before and during the Civil War. Nowadays we mostly hear from the Confederate side, as the losing side always remembers best and longest. I was ignorant of much of it myself until I learned about some of it from local newspapers and museums while spending summers in my father's homeland in upstate New York.
It would be like trying to get NeoConfederates to have sympathy for the Union and embrace Yankees. If the Confederacy had abolished slavery, I would be much more sympathetic, especially if they did it before the Union did. "Would have," "could have," don't mean much to me.
Besides, like I said, some of these Neorebs want to re-institute slavery after they secede. Some claim it will be "voluntary slavery," which is an oxymoron. They even tried to elbow their way into the libertarian Free State Movement, but the leader of it rejected their calls for eventual "voluntary slavery" and a secessionist theo-fascist state government in New Hampshire. They tried to get a secession movement going in South Carolina. I wonder how that's going.
I welcome whatever support those nostalgic for the Confederacy give libertarianism, but when they start to try to undermine and coopt the movement and rail on about the blessings of voluntary slavery, establishment of the Ten Commandments as the law of the land, stoning of gays and stuff like that, they lose me. Luckily, you're not one of those true radical NeoConfederates. You've had too much good influence there in the UK and you recognize the harm that slavery does to the slavemasters as well as the slaves. I've had NeoConfederates tell me that slavery was the best thing that ever happened to Africans and a blessed institution.
A side note of interest, the Civil War wasn't just a political war, it was also a war between peoples of different religions, cultural and ethnic/tribal backgrounds. A cousin of mine recently pointed out the religious aspect of it. The states lined up very neatly along religious lines and those differences persist to this day, though they are becoming watered down by homogenizing secular modern culture and mobility. Florida is rapidly becoming a culturally Northeastern state, for example. I have some sympathy for the dying Southern cracker culture.
I even have some sympathy for at least one reb--Stand Waitie. Interesting story, his.